1. Field of the Invention
The present invention generally relates to image forming apparatuses capable of feedback-controlling the transport speed of an endless belt member.
2. Description of the Related Art
In the field of image forming apparatuses such as copy machines, printers, and facsimile machines, there is an increasing need for the capability to produce high-quality color images as well as increasing the speed of the image formation process. Such a need may be addressed by a tandem-type color image forming apparatus equipped with image forming units for the individual colors of yellow, cyan, magenta, and black. In the tandem-type color image forming apparatus, toner images of the multi-colors are successively transferred onto an endless belt member, such as an intermediate transfer belt or a recording material transport belt on which a recording material is placed, one toner image over another. In such a tandem-type image forming apparatus, even if the drive source, such as a motor, is rotating at a constant speed, the endless transport speed of the endless belt member may vary if there is a variation in the thickness of the endless belt member, or if there is eccentricity in a belt drive roller or a drive gear engaged with the drive roller. As a result, a positional error may be caused between the overlapped toner images of the multi-colors, thus producing a color shift, small changes in color in a resultant printed image, or other forms of degradation in image quality.
In order to overcome such problems, one method may involve attaching a detector, such as an encoder, to the shaft of a driven roller that supports the endless belt member. In another method, a scale may be attached to the surface of the endless belt member and read by a detector. Based on the result of detection by such detectors, the endless transport speed of the endless belt member is detected and supplied for feedback control of the drive speed of the drive source for the endless belt member (see Japanese Laid-Open Patent Application Nos. 2004-220006 and 2005-115339). Because such a drive control method is capable of detecting speed variation components of the endless belt due to its thickness variation or the eccentricity in the belt drive roller or the drive gear, the speed variation component may be cancelled by performing an appropriate feedback control. Thus, the above conventional methods may enable the endless transport speed of the endless belt member to be maintained at a constant speed with accuracy, thus effectively overcoming the aforementioned problems.
In another image forming apparatus, cost reduction is achieved by reducing the number of components by driving the latent image carrier, such as a photosensitive drum, and the endless belt member using one drive unit (see Japanese Laid-Open Patent Application No. 2006-316985, for example).
However, when a drive control operation (“belt feedback control”) for maintaining a constant endless transport speed of the endless belt member is performed in the system where the endless belt member and the latent image carrier are driven by a single drive unit, the following problem may arise.
During the belt feedback control, a constant endless transport speed of the endless belt member is maintained by cancelling a speed variation component of the endless belt member (due to the belt thickness variation or the eccentricity in the belt drive roller) by producing a speed variation of the opposite phase to the phase of the speed variation component in the drive speed of the drive source. Thus, the latent image carrier having no speed variation component is driven at the drive speed having the speed variation of the opposite phase. As a result, variation in the surface transport speed of the latent image carrier may be caused. If the speed variation component of the endless belt member has a large instantaneous speed variation component, the drive speed of the drive source may be instantaneously increased in order to cancel the instantaneous speed variation of the endless belt. As a result, a local image stretching or contraction may be produced in the obtained image, causing lines of reduced or increased color density or other forms of image degradation.
Generally, even if there is a surface transport speed variation in the latent image carrier, no image degradation due to the surface transport speed variation is caused if the period of such variation or an integer multiple of the period corresponds to the time (“latent-image-formation-to-transfer time interval”) it takes for a latent image portion formed on the latent image carrier surface is moved to a transfer position of the endless belt member as the surface of the latent image carrier moves. This is due to the following.
When the surface transport speed is low, a latent image formed on the latent image carrier is stretched in a direction of movement of the latent image carrier surface (sub-scan direction). Conversely, when the surface transport speed of the latent image carrier is high, a latent image formed on the latent image carrier is contracted in the sub-scan direction. In contrast, when the surface transport speed is low, a toner image transferred onto the endless belt member or a recording material is contracted in the sub-scan direction. When the surface transport speed of the latent image carrier is high, a toner image transferred onto the endless belt member or the recording material is stretched in the sub-scan direction. When the period of the surface transport speed variation of the latent image carrier corresponds to the latent-image-formation-to-transfer time interval, a toner image corresponding to a latent image that is formed on the latent image carrier when the surface transport speed of the latent image carrier is low is transferred onto the endless belt member or the recording material when their surface transport speed is similarly low. Conversely, a toner image corresponding to a latent image that is formed on the latent image carrier when the surface transport speed is high is transferred onto the endless belt member or the recording material when the surface transport speed is similarly high.
As a result, the toner image corresponding to the latent image formed in an stretched condition is formed on the endless belt member or the recording material in a contracted condition with the corresponding scale ratio. Similarly, a toner image corresponding to a latent image formed in a contracted condition is formed on the endless belt member or the recording material in an stretched condition with the corresponding scale ratio. Thus, no stretching or contraction is caused in the resultant image due to the surface transfer speed variation of the latent image carrier, so that the aforementioned lines of image degradation do not occur.
However, the speed variation component of the endless belt member that would cause the aforementioned instantaneous speed variation may have a relatively long period, such as the period of the endless belt member or an integer multiple of the period of the endless movement of the endless belt, due to the engaging or disengaging of a component with the endless belt member. Practically, it is very difficult to design the apparatus such that the latent-image-formation-to-transfer time interval corresponds to such a relatively long period for various technical constraints. As a result, lines of image degradation may be caused in a printed image obtained by transferring the toner image from the latent image carrier at the time of the instantaneous speed variation of the endless belt member. In particular, if the belt feedback control for cancelling an instantaneous speed variation of the endless belt member is performed when the latent image formation on the latent image carrier and the toner image transfer from the latent image carrier are performed simultaneously, lines of image degradation may be caused at two locations per such speed variation, thus resulting in more serious image degradation.
The speed variation component that causes an instantaneous speed variation in the endless belt member may be caused regardless of the endless movement period of the endless belt member, such as by impact of the recording material with the endless belt member. For such an irregular speed variation component, the apparatus cannot be designed such that the latent-image-formation-to-transfer time interval corresponds to the period of such an irregular speed variation component, resulting in the lines of image degradation.
In one technology being developed by the present inventors, the endless belt member and the latent image carrier for a single-color image formation operation (“single-color latent image carrier”) are driven by a single drive unit while the other latent image carriers (multi-color latent image carriers) are driven by a separate drive unit. In accordance with this technology, belt feedback control is performed in a multicolor image formation operation (second operating mode) in which two or more colors of toner images are overlapped but not in the single-color image formation operation (first operating mode) in which there is no superposing of toner images. Instead, in the first operating mode, the drive source is operated at a constant speed.
In accordance with this technology, the endless transport speed of the endless belt member is maintained at a constant speed with high accuracy during the multicolor image formation operation, so that the toner images of the multi-colors can be accurately overlaid upon one another without color displacement, thus preventing color displacement or small color changes in the printed image. In this case, because a speed variation is caused in the single-color latent image carrier due to the belt feedback control, color displacement may be caused to some extent between the single-color latent image carrier and the multi-color latent image carriers in which there is no color displacement and the like. However, of the speed variation component of the endless belt member, those due to the eccentricity in the belt drive roller or a drive gear engaged with the belt drive roller or the belt thickness variation have a relatively short period, so that the apparatus can be designed such that the period corresponds to the latent-image-formation-to-transfer time interval, thereby eliminating the color displacement due to the speed variation components.
On the other hand, with regard to the speed variation component of the endless belt member having a relatively long period (such as a belt thickness variation whose period corresponds to the entire track of the endless belt member), the amount of color displacement is smaller than in the case of the short period, so that the impact of color displacement and the like is minor. This is due to the fact that, in the case of a speed variation component having a very long period of six times or more than the latent-image-formation-to-transfer time interval, for example, the amount of change in the surface transport speed of the latent image carrier as a result of the belt feedback control for cancelling the speed variation component is very small in the period in which a latent image portion formed on the latent image carrier surface is moved to the transfer position of the endless belt member. Thus, the above technology is capable of preventing image degradation such as color displacement during a multicolor image formation operation.
Further, in accordance with this technology, in the single-color image formation operation, the surface transport speed of the single-color latent image carrier does not vary depending on the speed variation component of the endless belt member but remains constant, so that the image-degrading lines can be prevented during the single-color image formation operation. At this time, because the speed variation component of the endless belt member is not cancelled, image stretching or contraction corresponding to the endless transport speed variation of the endless belt member may be caused in the printed single-color image, resulting in some density irregularities. However, such density irregularities in the single-color image may also be caused when belt feedback control is performed in both a multicolor image formation operation and a single-color image formation operation. In addition, such density irregularities are minor compared to the image-degrading lines. Therefore, the technology can achieve improved image quality as regards to the single-color image compared to the case where belt feedback control is performed in both the multicolor image formation operation and the single-color image formation operation, because of the elimination of the lined image degradation.
However, research conducted by the present inventors has indicated that the aforementioned technology has the following problems. The temperature in the image forming apparatus greatly varies depending on the status of use of the apparatus. For example, the temperature increases in a continuous image formation operation, or it decreases when no image formation operation is performed for a long time. Such temperature changes cause a change in the diameter of the belt drive roller or the thickness of the belt due to thermal expansion. For example, when the diameter r of the belt drive roller is increased by thermal expansion, the endless transport speed V of the endless belt member may increase even if the input of rotary angular speed ω into the belt drive roller is constant because of the relationship V=rω. Conversely, as the diameter r of the belt drive roller decreases, the endless transport speed V of the endless belt member decreases even if the input of rotary angular speed ω into the belt drive roller is constant. The same principle applies when the belt thickness changes due to thermal expansion, or when a diameter or thickness change occurs in the roller or the belt due to a change in humidity in the case of certain types of roller or belt material.
In the aforementioned technology, no belt feedback control is performed during the single-color image formation operation, so that the change in the endless transport speed (average speed) of the endless belt member due to thermal expansion is not corrected during the single-color image formation operation. Thus, in the single-color image formation operation, the average speed of the endless belt member may vary depending on the temperature change in the image forming apparatus. As a result, the single-color image printed on a recording material may be shifted in the sub-scan direction, or the single-color image as a whole may be stretched or contracted.
This problem similarly occurs in an image forming apparatus in which the endless belt member and the latent image carriers are driven by a single drive unit, where drive source feedback control and belt feedback control are selectively performed. In the drive source feedback control, the drive speed of the drive unit is controlled to a constant speed based on a first detection signal obtained by detecting the rotating speed of the rotating drive force supplied by the drive unit to the endless belt member. In the belt feedback control, the drive speed of the drive unit is controlled to a constant speed based on a second detection signal obtained by detecting the endless transport speed of the endless belt member.
Thus, in such an image forming apparatus, no belt feedback control is performed when drive source feedback control is performed, so that the change in the endless transport speed (average speed) of the endless belt member due to the aforementioned temperature change is not corrected. As a result, an image printed on the recording material may be shifted in the sub-scan direction, or the entire image may be stretched or contracted.